Passive limiter for high-frequency waves

ABSTRACT

A passive limiter, designed to block transmission of highamplitude pulses over a signal path such as a duplexing channel of a radar transceiver, comprises two or more pairs of diodes connected in antiparallel relationship between ground and a common point on the transmission channel. Each diode consists of three semiconductor layers, i.e. two heavily doped outer layers N , P and a lightly doped intermediate layer N or P. Each diode is enclosed in a thermally conductive housing with two spaced-apart end walls receiving the semiconductive wafer therebetween, the wafer resting on one end wall and being tied to the opposite end wall by one or more leads leaving a large void inside a dielectric shell of, for example, beryllium oxide.

United States Patent m1 Henry et al.

[ Oct. 23, 1973 Assignee: Thomson-CSF, Paris, France Filed: Apr. 3, 1972 Appl. No.1 240,743

[30] Foreign Application Priority Data Apr. 9, 1971 France 7112746 US. Cl. 333/17, 333/7, 333/81 B, 333/84 R int. Cl. 1101p 1/22, H04b 3/04 Field of Search 333/81 R, 81 A, 81 B, 333/84 M, 17, 7; 317/234 R, 234 A References Cited UNITED STATES PATENTS 3,346,825 10/1967 Scott et al 317/235 AD 3,517,272 6/1970 Lee et al. 333/84 M 3/1971 Friedman 333/7 OTHER PUBLICATIONS Ekinge et al. A New Microwave Variable Attenuator" in [BE Transactions on Microwave Theory and Techniques Sept. 1970 VOLMTT18; pp. 661-662.

Primary Examiner-Rudolph V. Rolinec Assistant Examiner-Marvin Nussbaum Attorney-Karl F. Ross [57] ABSTRACT A passive limiter, designed to block transmission of high-amplitude pulses over a signal path such as a duplexing channel of a radar transceiver, comprises two or more pairs of diodes connected in antiparallel relationship between ground and a common point on the transmission channel. Each diode consists of three semiconductor layers, i.e. two heavily doped outer layers N P and a lightly doped intermediate layer N or P. Each diode is enclosed in a thermally conductive housing with two spaced-apart end walls receiving the semiconductive wafer therebetween, the wafer resting on one end wall and being tied to the opposite end wallby one or more leads leaving a large void inside a dielectric shell of, for example, beryllium oxide.

7 Claims, 16 Drawing Figures nan-y Our present invention relates to a passive limiter for high-frequency waves such as those used in radar transmission.

In a radar system using a common antenna for transmission and reception, it is necessary to block the signal channel between the antenna and the receiver against high-amplitude pulses generated by the associated transmitter. Protective circuitry of this kind is also needed where the transmitter may be harmed by strong signals picked up from nearby sources such as other radar transmitters. Particularly in the latter case, where the timing of the high-amplitude pulses is not known in advance, the protective circuitry should respond instantly and automatically to signal voltages exceeding a predetermined threshold.

Conventional limiters used for this purpose include so-called TR tubes which are inserted between two couplers of the 3-dB type in a shunt path and, in response to voltages surpassing their breakdown potential, fire to establish an effective short circuit which reflects the incoming waves. While these tubes operate generally satisfactorily, they have a limited service life which after a while allows the transmitted voltages to attain excessive levels detrimental to the'receiver. Another drawback of these tubes is their delayed firing which, lets part of the incoming wave energy reach the receiver with possible harmful consequences.

1 The general object of our invention is to provide a limiter of this character which avoids the aforestated drawbacks and, while being highly discriminative between high and low voltages, causes sharp attenuation of the former while permitting transmission of the latter without significant loss. 4

More particularly, our invention aims at providing a passive limiter capable of absorbing high-frequency power peaks on the order of several kW, with an average power exceeding 100 watts, while decoupling the signal source from the load with an attenuation upward of 40 dB.

These objects are realized, in accordance with our invention, by the provision of a pair of voltage-limiting semiconductive diodes connected in antiparallel relationship between a' common junction on the transmissionchannelto be protected and apoint of fixed potential, preferably .ground, these diodes having substantially identical forward threshold'potentials defining the limits of a range of bipolar voltages which are to be freely transmitted over that channel.

According to a more specific feature of our invention, each diode comprises a stack of semiconductive layers (preferably of silicon) including two relativelyheavily doped outlet layers of opposite conductivity types, hereinafter designated P and N*, and a relatively lightly doped intermediate layer of one conductivity type, designated P or N. Such a diode, accordingly, may be identified by the symbols N -N-P or P*'- P-N. In contradistinction to the known P-I-N diodes with inert intermediate layer, which require the application of a biasing voltage to facilitate conduction, our new diodes with three active layers need not be primed and can therefore be used as protection against highamplitude pulses arriving at unpredictable times. Also, their dynamic effectiveness substantially exceeds that of P-l-N diodes (which generally cannot introduce an configurations under special circumstances. Thus, the

break-down threshold may be raised by cascading two or more diodes of the same conductivity type, whereas protection over a wider frequency band may necessitate the use of several diode pairs spaced along the transmission channel.

Another feature of our invention relates to the mounting of each diode (or possibly of several stacked diodes) in a thermally conductive housing facilitating the dissipation of the generated heat. Such a housing advantageously includes two electrically conductive end walls, one of them supporting the wafer constituted by the semiconductive layers while the other is held spaced therefrom by a surrounding wall of dielectric and highly heat-conductive material, such as beryllium oxide (BeO). If the signal channel is astrip conductor,

one of the end walls of each diode housing maybe anode or cathode lead. I

If the channel has the form of a waveguide, the diodes may be mounted within the guide structure to which they may be electrically connected with the aid of an intermediate conductive body tied to a neutral point of the structure.

The above andother features of our invention will be described in detail hereinafter with reference to the accompanying drawing in which: r

FIG. 1 isa diagrammatic view of a voltage-limiting diode corresponding to our invention;

FIGS. 2 and 3 are elevational views, partly in section, of two modes of mounting such a diode in a housing;

.FIG. 4 is afragmentary circuit diagram showing a pair of such diodes connected to a signal channel;

FIG. 5 is a graph showing the current. flow through the diode of FIG. 1;

FIG. 6. is an equivalent circuit diagram for a limiting diode in a transmitting circuit;

FIG. 7 is a graph showing the current flow through a pair of' diodes as illustrated in FIG..4; ,FIG. 8 is a sectional view of an assembly including pair of diodes with housings as shown in FIGS. 2 and 3;

FIG. 9 is a circuit diagram similar to-FIGA, showing two. pairs of diodes; 1 1 1 v FIG.'10 is an equivalent circuit similar to FIG. 6, relating'to the condition in which the diode is subjected to low-amplitudesignals; I a FIG. 1 l is a similar equivalent circuit for a diode subjected to high-amplitude signals;

FIG. 12 is a diagrammatic perspective view of a waveguide incorporating a protective network with two diodes according to our invention;

FIG. 13 is a view similar to FIG. 4, showing two sets. of multiple diodes, I

FIG. 14 is a circuit diagram of a duplexer including a pair of limiters according to the invention; and

FIGS. 15 and16 are diagrams similar to FIG. 14, showing different diode combinations included in the limiters of the latter Figure. i

In FIG. 1 we have shown a conductive base 1 serving as a cathode connection for a diode in the form a wafer 5 of semiconductive material, specifically .silicon,

formed from three stacked layers 2, 3 and 4. Bottom layer 2 is of the type N* and may have a thickness on the order of 60 microns. Intermediate layer 3 is of type N and is considerably thinner than layer 2 which it partly overlies. Top layer 4, coextensive with layer 3, is still thinner than the latter end of type 1.

FIG. 2 shows the wafer inside a housing whose lower part forms an electroconductive end wall integral with cathode l. The top layer 4 (FIG. 1) of wafer 5 is connected by wires 9 to an opposite end wall 7 of similar conductive material constituting the anode of the diode assembly. The two conductive members 7 and 10 are separated by a peripheral wall 8 of dielectric but highly heat-conductuve material such as BeO. The entire unit has been designated 11.

A similar unit 12, illustrated in FIG. 3, has the same basic construction except that wafer 5 is now mounted on the top wall serving as a cathode 1; anode 7 is integral with the bottom part 10 of the unit.

Two units as shown in FIGS. 2 and 3, if connected to a common junction point at corresponding ends (e.g. at the top) and grounded at their opposite ends as illustrated, form an antiparallel pair (see FIG. 4) responsive to voltages of. opposite polarities. A similar pair can be made up from two identical units 11 or 12 if their conductivity types are relatively inverted, as by making the layers 2, 3 and 4 of FIG. 1 of type P P and N respectively. In subsequent Figures, the embodiments described with reference to these diode combinations could'be interchangeably employed.

As illustrated in FIG. 4, two such units 11 and 12 are connected in a pair of parallel paths but with relatively inverted polarities (a relationship termed antiparallel") between ground and a junction point 6 on a line 13 which may be a strip conductor as more fully illustrated in FIG. 8. These diodes are virtually nonconductive for low signal voltages of high frequency passing along the channel 13; in the presence of higher voltages surpassing their forward threshold potential V; (FIG. 5), however, the diode of theproper polarity breaks down and becomes a short circuit, causing the incoming wave to be reflected toward its origin.

The potential V, illustrated in FIG. 5 may be on the I order of 1 V and corresponds to the voltage drop across the forbidden band of the semiconductive material (silicon). A diode as described with reference to FIG. 1 may sustain a peak'power on the order of 1.3 kW and voltage peaks of about 720 V. Its equivalent circuit, shown in FIG. 6, comprises a shunt resistance R, across a signalsource '15 working into a line resistance 16 (e.g. of ohms); shunt resistance R, includes such parasitic impedan'ces as the capacitance of the diode housing and the inductivity of the leads 9.

Let us consider an input voltage V sincot where o) is the pulsatance of a radar frequency of about 3 GHz. This corresponds to a cycle length T (FIG. 5) of about 0.33 ns, which is a small fraction of the recombination period of the minority and majority charge carriers (on the order of I00 ns) injected into the diode by the impressed signal voltage. It may be assumed that the first half-cycle of any incoming signal lies below the break-down potential V; so that the diode does not conduct and the carriers are swept out at the next halfcycle. Upon a subsequent rise of the voltage V beyond the forward threshold V,, however, e.g. to a value of several volts, there is developed across the diode a voltage drop equal to V, which opposes the applied voltage on the forward swing'and aids it on the reverse swing during which it persists until the charge carriers have been swept out. The total diode current, composed of phases I, (forward) and I,- (reverse), is zero from the instant t O to the time t t when the diode ceases to conduct.

The diode voltage V, V sinwt, at the instant of blocking can be calculated by integration from O to T/2 and from T/2 to T, with V, V, In the specific case here considered, V, equals about 95 V.

With the paired diodes shown in FIG. 4, however, the voltage V, cannot develop across one diode since the other diode conducts in the forward direction and therefore limits the voltage of junction point 6 to a value V,= R,I. After several tens of cycles, thus well before the power attains its peak value, steady-state operation sets in. At this stage the currents traversing each diode have the shape shown in FIG. 7.

The disclosed arrangement prevents the appearance of detrimental voltage peaks inasmuch as the sweepout time for the charge carriers'of one diode equals the injection time for the other diode. Thus, the terminal voltageof the forwardly driven diode is substantially limited to V) as the flooding of its intermediate layer with charge carriers makes its break-down resistance R, extremely low.

As more fully illustrated in FIG. 8, the conductive housing portions of the two units 11 and 12, contact a conductor strip 13 from opposite sides, the strip being sandwichedbetween two. grounded conductor shields 17 and 18 with interposition of insulating members 39, 40 surrounding these units with clearance. Shields 17, 18 are perforated to receive two cylindrical sleeves 21, 22 with threaded bushings 41, 42 accommodating respective inserts 19 and 20 which in turn hold the units 11 and 12.. A loop ST extends from central conductor 13 to groundon member 17 so as to act as aninductance resonating the leakage capacitance C, (FIG. 10) of the diode housings at the'operating frequency.

With the units 11 and 12 mounted in relatively inverted position, their wafers 5 have the same orientation'as will be apparent from FIGS. 2 and 3; thus, the applied field E acts with relatively inverted polarity upon thetwo diodes. The junction capacitance C, of the diodes at applied zero voltage may range between 0.4 and 0.55 pF. In a frequency band of 2,450 to 2,500

MI-Iz the standing-wave ratio (SWR) of the system is.

' flected wave-with reference to the incident wave ison the order of '25 dB.

As schematically illustrated in FIG. 9, a protective network according to our invention may also comprise two limiter stages 23, 24 each including a pair of units 1 l and 12 connected in antiparallel relationship as defined above, their junction points 6, 6' being spaced by a'quarter wavelength (M4) of the oscillations emitted by source 15. Channel 13 may again be a strip conductor of the type illustrated in FIG. 8. The presence of the second stage 24, downstream of stage 23, provides improved low-level impedance matching, increases the operative bandwidth and accordingly allows a greater margin for deviations of the units from a predetermined standard. With proper matching the low-level losses are still small, e.g. of about 0.6 dB.

In the high-voltage condition, as shown in FIG. 11, I

the large zero-level resistance R is replaced by the small shunt resistance R, in series with the internal inductance L,; the parasitic reactances C, and ST may be ignored in this situation. The attenuation of the reflected wave, due primarily to the upstream diode pair 23, is on the order of 0.3 dB.

Tests carried out with a limiter as shown in FIG. 9, for operating frequencies again ranging between 2,450 and 2,550 MHz, revealed an SWR of 1.20 or less; attenuation began at 25 dB, for a mean power of 1 mW and peaks of 20 mW, rising to 43 dB for a mean power of 80 W and peaks of 1.6 kW.

FIG. 12 illustrates the possibility of employing our protective diodes in a waveguide 25 serving as a transmission channel for microwaves to be selectively attentuated. In this instance, where the problems of heat dissipation do not manifest themselves in the same way as with metallic conductors, two three-layer diodes 26, 27 may be mounted without thermally conductive enclosures in the electric plane (vector E) of the rectangular guide, between opposite walls thereof and confronting surfaces of a conductive insert 28 of brass or the like which in turn is tied by conductors 29, 30 to the neutral midpoints on the two other guide walls. Thus, the broad bases (1, FIG. 1) of the diodes may rest directly on the top and bottom walls of the waveguide while their exposed layers (4) bear upon the body 28, the two diodes being of the same conductivity type but opposite orientation relative to field E.

FIG. 13 illustrates the possibility of cascading several diodes ll, 12 in two antiparallel stacks between ground and the junction point 6 on channel 13. This arrangement enables the absorption of still greater signal powers but calls for more intensive cooling.

In FIG. 14 we have illustrated a duplexer for a radar station including a transceiving antenna 33, a transmitter 34, a dummy load 35 and a receiver 36 conventionally interconnected by channels 43, 44 with the aid of two 3-dB couplers 31, 32 in tandem with each other. Between these couplers, each channel is provided with a limiter according to our invention designated 37 and 38, respectively. As shown in FIG. 15, each of these limiters is of the two-unit type illustrated in FIG. 4; FIG. 16 shows an arrangement as discussed with reference to FIG. 9, i.e. two pairs of diodes spaced a quarter wavelength apart. A tested embodiment of the system of FIG. 15, operating in the aforementioned frequency band, had a leakage loss of less than 0.4 dB during both transmission and reception; the decoupling effect between transmitter and receiver had a substantially constant value upward of 43 dB for mean powers ranging from 20 to 120 watts with peaks up to 2.5 kW. In the system of FIG. 16 this decoupling effect was found to be in excess of 47 dB.

The attenuation of strong signals from nearby transmitters was established at approximately 25 dB for the system of FIG. 15 and at roughly that for the pulses from the associated transmitter 34 (i.e. better than 47 dB) with the system of FIG. 16. The performance especially of the simpler system according to FIG. 15 could, if desired, be further improved by the insertion of a supplemental passive limiter of low attenuation level (eg about 15 dB) in the high-frequency stage of the radar receiver 36.

We claim 1. A protective structure for a signal channel carrying high-frequency electromagnetic waves, comprising:

a pair of alignedly juxtaposed heat-dissipative housings each including two spaced-apart electrically conductive first and second end walls and a dielectric peripheral wall of high thermal conductivity surrounding the space between said end walls, the first end wall of one housing being physically and conductively secured to the first end wall of the other housing;

mounting means physically and conductively secured to said housings by their second end walls, said mounting means forming a clearance aboutsaid housings;

a voltage-limiting semiconductive diode in each of said housings physically supported on.one end wall thereof and provided with a conductive connection to the opposite end wall; and A circuit means connecting said channel between said mounting means and said first end wall of each housing.

2. A structure as defined in claim 1 wherein each diode comprises a semiconductive wafer supported on said one end wall of the associated housing in spaced relationship with said opposite end wall, said conductive connection including wire means extending from said wafer to said opposite end wall.

3. A structure as defined in claim 1 wherein said channel comprises a strip conductor sandwiched between the first end walls of said housings.

4. A structure as defined in claim 1 wherein said dielectric wall consists of beryllium oxide.

5. A structure as defined in claim 1 wherein each of said diodes comprises a stack of semiconductive layers including two relatively heavily doped outer layers of opposite conductivity types and a lightly doped intermediate layer of one conductivity type.

6. A structure as defined in claim 5 wherein said layers consist of doped silicon.

7. A protective structure for a signal channel carrying high-frequency electromagnetic waves, said channel including a rectangular waveguide, comprising:

a metallic body suspended midway within said waveguide and conductively connected to electrically neutral points on opposite walls of said waveguide; and

a pair of voltage-limiting semiconductive diodes disposed in the electric plane of said waveguide, said diodes being connected with relatively inverted polarity between the other two walls of said waveguide and confronting surfaces of said body. 

2. A structure as defined in claim 1 wherein each diode comprises a semiconductive wafer supported on said one end wall of the associated housing in spaced relationship with said opposite end wall, said conductive connection including wire means extending from said wafer to said opposite end wall.
 3. A structure as defined in claim 1 wherein said channel comprises a strip conductor sandwiched between the first end walls of said housings.
 4. A structure as defined in claim 1 wherein said dielectric wall consists of beryllium oxide.
 5. A structure as defined in claim 1 wherein each of said diodes comprises a stack of semiconductive layers including two relatively heavily doped outer layers of opposite conductivity types and a lightly doped intermediate layer of one conductivity type.
 6. A structure as defined in claim 5 wherein said layers consist of doped silicon.
 7. A protective structure for a signal channel carrying high-frequency electromagnetic waves, said channel including a rectangular waveguide, comprising: a metallic body suspended midway within said waveguide and conductively connected to electrically neutral points on opposite walls of said waveguide; and a pair of voltage-limiting semiconductive diodes disposed in the electric plane of said waveguide, said diodes being connected with relatively inverted polarity between the other two walls of said waveguide and confronting surfaces of said body. 